1. Field of the Invention
The present invention relates to a ferromagnetic tunnel junction; and a magnetic head, a magnetic recording device, and a magnetic memory device that use the ferromagnetic tunnel junction.
2. Description of the Related Art
Ferromagnetic tunnel junctions including a pair of ferromagnetic layers separated by a tunnel insulating film have been proposed for use as magnetic sensors for magnetic heads and the like. Since these ferromagnetic tunnel junctions are expected to show a very large magnetoresistance change in response to a faint change in magnetic fields, they are promising for use as ultrasensitive magnetic sensors for magnetic heads.
FIGS. 1A and 1B illustrate the principal of such a ferromagnetic tunnel junction 100. Referring to FIGS. 1A and 1B, the ferromagnetic tunnel junction 100 comprises ferromagnetic layers 101 and 103 of NiFe or Co, and a tunnel insulating film 102 of AlOx with a thickness of several nano meters interposed between the ferromagnetic layers 101 and 103. Electrons with an upward spin direction and electrons with a downward spin direction generate a tunneling current flowing through the tunnel insulating film 102 in a direction perpendicular to a principal surface thereof.
FIG. 1A shows a state in which substantially no external magnetic field is present. The ferromagnetic layer 101 is a pinned magnetic layer whose magnetization direction is fixed by an antiferromagnetic layer (not shown) disposed in contact with the lower side of the ferromagnetic layer 101. The ferromagnetic layer 103 is a free magnetic layer whose magnetization direction changes depending on the direction of external magnetic fields. When substantially no external magnetic field is applied, the magnetization of the ferromagnetic layer 103 (herein after referred to as “free magnetic layer 103”) is oriented in the same direction of a magnetic easy axis thereof. In this state, the magnetization directions of the ferromagnetic layer 101 (hereinafter referred to as “pinned magnetic layer 101”) and the ferromagnetic layer 103 (hereinafter referred to as “free magnetic layer 103”) are parallel to each other. In contrast, when an external magnetic field H is applied as shown in FIG. 1B, the magnetization of the free magnetic layer 103 is oriented in the same direction as the external magnetic field H so as to be antiparallel to the magnetization of the pinned magnetic layer 101.
In the ferromagnetic tunnel junction 100 having the configuration as described above, tunneling probability of the tunneling current varies depending on the magnetization state of the pinned magnetic layer 101 and the free magnetic layer 103. Therefore, tunneling resistance R of the ferromagnetic tunnel junction 100 varies due to the external magnetic field H, and is expressed as the following Equation (1):R=Rs+(½)ΔR(1−cos θ)  (1)wherein Rs represents the tunneling resistance in a state where the magnetization directions of the pinned magnetic layer 101 and the free magnetic layer 103 are parallel to each other; θ represents the angle formed by the magnetization of the pinned magnetic layer 101 and the magnetization of the free magnetic layer 103; and ΔR, which is always positive, represents the difference in the tunneling resistance between the state where magnetizations of the pinned magnetic layer 101 and the free magnetic layer 103 are parallel to each other and the state where magnetizations of the pinned magnetic layer 101 and the free magnetic layer 103 are antiparallel to each other. Also, the change ratio of tunneling resistance, i.e, the TMR ratio is defined as ΔR/Rs.
According to Equation (1), the tunneling resistance R is minimized when the magnetizations of the pinned magnetic layer 101 and the free magnetic layer 103 are parallel to each other, and is maximized when antiparallel to each other. This change of the tunneling resistance R results from the presence of the electrons with the upward spin direction (up-spin electrons) and the electrons with the downward spin direction (down-spin electrons) in electronic current. Generally, a nonmagnetic body has the same number of up-spin electrons and down-spin electrons, and therefore does not exhibit magnetism as a whole. On the other hand, a ferromagnetic body has different numbers of up-spin electrons and down-spin electrons, and therefore exhibits upward or downward magnetism as a whole.
When an electron tunnels between the pinned magnetic layer 101 and the free magnetic layer 103, the electron retains the spin state before and after the tunneling. This means that when electrons tunnel from the free magnetic layer 103 to the pinned magnetic layer 101, a vacant energy level corresponding to the spin state of the electron is present in the pinned magnetic layer 101. If there is no vacant energy level, the tunneling of electrons does not occur.
The TMR ratio ΔR/Rs is a product of spin polarizability of the source (the free magnetic layer 103) and polarizability of the vacant energy level of the target (pinned magnetic layer 101), and is represented as:ΔR/Rs=2P1P2/(1−P1P2)  (2)wherein P1 represents the spin polarizability of the free magnetic layer 103, and P2 represents the spin polarizability of the vacant energy level of the pinned magnetic layer 101. P1 and P2 are calculated as follows:P1, P2=2(Nup−Ndown)/(Nup+Ndown)wherein Nup represents the number of up-spin electrons or the number of levels for the up-spin electrons, and Ndown represents the number of down-spin electrons or the number of levels for the down-spin electrons.
The spin polarizability P1, P2 generally depends on the type of ferromagnetic materials. If a proper material is chosen, the spin polarizability may reach close to 50%. A magnetic sensor using such a ferromagnetic tunnel junction is therefore expected to have a magnetoresistance ratio of several dozen percent, which is much greater than that of AMR (anisotropic magnetoresistive) and GMR (giant magnetoresistive) magnetic sensors. Accordingly, magnetic heads using ferromagnetic tunnel junctions are considered advantageous for use in super-high magnetic recording and reproduction (see, for example, Patent Document 1).
Recently, a ferromagnetic tunnel junction with a multilayer body of Fe(001)/MgO(001)/Fe(001) has been presented, in which a tunnel insulating film is formed of magnesium oxide and ferromagnetic layers are formed of single-crystal Fe (see Non-Patent Document 1). This multilayer body is epitaxially grown by an epitaxial method. It is reported that this ferromagnetic tunnel junction exhibits a TMR ratio of 200% or higher at room temperature.
According to another study, a ferromagnetic tunnel junction with a multilayer body of CoFe/MgO(001)/CoFe, in which amorphous CoFe is used as a material of ferromagnetic layers in place of single-crystal Fe, exhibits a TMR ratio of 220% at room temperature (see Patent Document 2). It is also reported that a ferromagnetic tunnel junction with ferromagnetic layers formed of amorphous CoFeB in place of amorphous CoFe demonstrates a remarkably high TMR ratio (see Patent Document 3). Since amorphous CoFe films, amorphous CoFeB films, and MgO films can be formed by sputtering, these ferromagnetic tunnel junctions are readily manufacturable using conventional magnetic head manufacturing process.
[Patent Document 1] Japanese Patent No. 2871670
[Non-Patent Document 1] Yuasa et al. “Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions” Nature Materials vol. 3, pp. 868-871 (2004)
[Non-Patent Document 2] Parkin et al. “Giant tunneling magnetoresistance at room temperature with MgO(100) tunnel barriers” Nature Materials vol. 3, pp. 862-867 (2004)
[Non-Patent Document 3] Tsunekawa et al. “Effect of capping layer material on tunnel magnetoresistance in CoFeB/MgO/CoFeB magnetic tunnel junctions” manuscripts of INTERMAG 2005, session No. HP-08 (Apr. 4, 2005)
However, if the ferromagnetic tunnel junction 100 shown in FIGS. 1A and 1B includes the free magnetic layer 103 of CoFeB, and if a ferromagnetic film of NiFe, CoFe, or the like is formed on the free magnetic layer 103 as disclosed in Non-Patent Document 3, TMR ratio drops significantly due to a heat treatment for improving the quality of the tunnel insulating film 102 compared to the case where such a ferromagnetic film is not present.
Also, in the case where the antiferromagnetic layer of the ferromagnetic tunnel junction 100 is formed of an ordered alloy film, such as a PdPtMt film, the TMR ratio may decrease due to a heat treatment for ordering the antiferromagnetic layer.